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Y. Cao, C. Huang, Q. Lu* __________ _________________________________________________________________
The development of strategies to access boronate esters from ubiquitous aliphatic C−H bonds is of long-standing interest in the synthesis community. Here photoelectrochemically driven C(sp3)−H borylation of alkanes is developed, in which iron, an abundant earth-based resource, is employed as a photoelectrochemical catalyst. Using this protocol, direct borylation of strong alkyl C−H bonds is efciently achieved at a low oxidation potential of ∼0.3 V and mild conditions. A wide range of structurally diverse alkyl boronic esters, including versatile α-silyl boronic esters, can be accessed with good regioselectivity.
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C. Huang#, Y. Tao#, X. Cao, C. Zhou, Q. Lu* __________ _________________________________________________________________
Asymmetric electrocatalysis offers exciting new strategies for the synthesis of chiral molecules through novel reaction pathways. However, simultaneous activation of reactants on both electrodes via asymmetric paired electrolysis, which is more energy efficient and economic than single half-electrode synthesis, remains a formidable challenge. Herein, an asymmetric olefin–sulfonylimine coupling via paired electrocatalysis is presented for the first time. In this protocol, Co-catalyzed hydrogen atom transfer on the anode and Ni-catalyzed sulfonylimine reduction on the cathode were seamlessly cross-coupled. The new catalytic system enables the formation of chiral amine products bearing a tetrasubstituted carbon stereocenter with a high enantioselectivity (up to 96% ee).
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Y. Tao, C. Huang, Q. Lu* __________ _________________________________________________________________
Enantioselective synthesis of chiral cyclobutanes via direct cycloaddition of C–C single bonds with C=C double bonds has remained an unmet challenge. Now, a photoelectrocatalytic system enabling asymmetric dehydrogenative [2+2] cycloaddition of alkyl ketones and alkenes has been developed.
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L. Zou, S. Xiang, R. Sun, Q. Lu* __________ _________________________________________________________________
We report a combination of electrocatalysis and photoredox catalysis to perform selective C(sp3)–H arylation/alkylation of alkanes, in which a binary catalytic system based on earth-abundant iron and nickel is applied. Reaction selectivity between two-component C(sp3)–H arylation and three-component C(sp3)–H alkylation is tuned by modulating the applied current and light source. Importantly, an ultra-low anodic potential (~0.23 V vs. Ag/AgCl) is applied in this protocol, thus enabling compatibility with a variety of functional groups (>70 examples). The robustness of the method is further demonstrated on a preparative scale and applied to late-stage diversification of natural products and pharmaceutical derivatives.
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L. Zou, X. Wang, S. Xiang, W. Zheng, Q. Lu* __________ _________________________________________________________________
Due to the intrinsic inertness of alkanes, strong oxidative conditions are typically required to enable their C(sp3)−H functionalization. Herein, a paired electrocatalysis strategy was developed by integrating oxidative catalysis with reductive catalysis in one cell without interference, in which earth-abundant iron and nickel are employed as the anodic and cathodic catalysts, respectively. This approach lowers the previously high oxidation potential required for alkane activation, enabling electrochemical alkane functionalization at the ultra-low oxidation potential of ~0.25 V vs. Ag/AgCl under mild conditions. Structurally diverse alkenes, including challenging all-carbon tetrasubstituted olefins, can be accessed using readily available alkenyl electrophiles.
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P. Peng, Y. Zhong, C. Zhou, Y. Tao, D. Li, Q. Lu* __________ _________________________________________________________________
Direct functionalization of inert C–H bonds is one of the most attractive yet challenging strategies for constructing molecules in organic chemistry. Herein, we disclose an unprecedented and Earth abundant Cu/Cr catalytic system in which unreactive alkyl C–H bonds are transformed into nucleophilic alkyl–Cr(III) species at room temperature, enabling carbonyl addition reactions with strong alkyl C–H bonds. Various aryl alkyl alcohols are furnished under mild reaction conditions even on a gram scale. Moreover, this new radical-to-polar crossover approach is further applied to the 1,1-difunctionalization of aldehydes with alkanes and different nucleophiles. Mechanistic investigations reveal that the aldehyde not only acts as a reactant but also serves as a photosensitizer to recycle the Cu and Cr catalysts.
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B. Wang, X. Zhang, Y. Cao, L. Zou, X. Qi, * Q. Lu* __________ _________________________________________________________________
This report describes the unprecedented electrooxidation of a solvent (e.g., DMF)-ligated B2cat2 complex, whereby a solvent-stabilized boryl radical is formed via quasi-homolytic cleavage of the B–B bond in a DMF-ligated B2cat2 radical cation. Cyclic voltammetry and density functional theory provide evidence to support this novel B–B bond activation strategy. Furthermore, a strategy for the electrochemical gem-diborylation of gem-bromides via paired electrolysis is developed for the first time, affording a range of versatile gem-diborylalkanes, which are widely used in synthetic society. Notably, this reaction approach is scalable, transition-metal-free, and requires no external activator.
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X. Zheng#, P. Peng#, C. Huang, Q. Lu* ___________________________________________________________________________
Sustainable and site-selective C-H functionalization is still a challenge because of the comparable properties of bonds of the same type. Here, an electrochemical C(sp2)-H/C(sp3)-H coupling with aryl iodides is reported for the first time. The C(sp2)-H bond outperformed the much weaker C(sp2)-I bond in participation in C(sp2)-H/C(sp3)-H coupling. This protocol features mild reaction conditions, good functional-group tolerance and scalability. Cross-coupling products with C-I bonds are versatile moieties for further synthetic manipulation, they can undergo Suzuki, Sonogashira, Ullmann-Ma, Catellani, and Heck reactions etc to introduce new functional groups into target molecules with good yields. Mechanistic investigations showed that the site selectivity of the reaction is enabled by an iodine-assisted [3,3]-sigmatropic rearrangement, and the interaction of hypervalent iodine generated in situ with an enolate is the rate-determining step.
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C. Huang, W. Ma, X. Zheng, M. Xu, X. Qi, * Q. Lu* ___________________________________________________________________________
Selective hydrogenation of
epoxides would be a direct and powerful approach for alcohol synthesis, but it
has proven to be elusive. Here, electrochemically epoxide hydrogenation using
electrons and protons as reductants is reported. A wide range of primary,
secondary, and tertiary alcohols can be achieved through selective Markovnikov
or anti-Markovnikov ring opening in the absence of transition metals.
Mechanistic investigations revealed that the regioselectivity is controlled by
the thermodynamic stabilities of the in situ generated benzyl radicals for
aryl-substituted epoxides and the kinetic tendency for Markovnikov selective
ring opening for alkyl-substituted epoxides.
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B. Wang#, P. Peng#, W. Ma, Z. Liu, C. Huang, Y. Cao, P. Hu, X. Qi, * Q. Lu* __________ _________________________________________________________________
Herein, a fast, scalable, and transition-metal-free borylation of alkyl halides (X = I, Br, Cl) enabled by electroreduction is reported. This process provides an efficient and practical access to primary, secondary, and tertiary boronic esters at a high current. More than 70 examples, including the late-stage borylation of natural products and drug derivatives, are furnished at room temperature, thereby demonstrating the broad utility and functional-group tolerance of this protocol. Mechanistic studies disclosed that B2cat2 serves as both a reagent and a cathodic mediator, enabling electroreduction of difficult-to-reduce alkyl bromides or chlorides at a low potential.
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Q. Lu#, S. Mondal#, S. Cembellín, S. Greßies, F. Glorius.* ___________________________________________________________________________
Herein we describe an unprecedented RuII-catalyzed site-selective and regiospecific annulation of benzoic acids with propargylic carbonates. The weakly coordinating carboxylic acid moiety outperformed other typically used directing groups in C–H activation, including ketone, nitrile, sulfonamide, amide and strongly coordinating nitrogen heterocycles. This is an important step towards the application of C–H activation reactions in complex (functional) real-world molecules.
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R. J. Perkins, R. Feng, Q. Lu, and Kevin D. Moeller.* __________ _________________________________________________________________
While many steps in an oxidative cyclization reaction can be important, it is the cyclization step itself that plays the central role. If this step does not proceed well, then optimization of the rest of the sequence is futile. We report here that the key to the cyclization is channeling the reaction down the correct pathway. Some reactions require the use of a radical pathway and some require the use of a radical cation pathway. An example of each is provided along with a strategy for accessing both pathways using a common intermediate.
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H. Wang, Y. Li, Q. Lu, M Yu, X Bai, S Wang, H Cong, H Zhang, A. Lei.* ___________________________________________________________________________
Site-selective intermolecular C–H bond functionalization is of central importance to synthetic chemistry. In particular, direct β-functionalization of N-heterocycles still remains a great challenge. Herein, we develop a strategy for oxidation-induced thiolation and selenation at the β-position of piperidine derivatives and 1,2,3,4-tetrahydroisoquinoline via C–H bond functionalization. Various 4-sulfenylisoquinolines, 3-sulfenylpyridines, and 4-selenylisoquinolines can be obtained by using O2 as the only oxidant. Notably, neither a directing group nor a metal catalyst is necessary in this transformation. The preliminary mechanistic studies revealed that the oxidation and rearrangement pathway were key steps in this transformation, which provides a meaningful strategy for controlling site selectivity in the β-functionalization of N-heterocycles.
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Q. Lu, S. Mondal, S. Cembellín, S. Singha, F. Glorius.* ___________________________________________________________________________
An unprecedented MnI/AgI-relay-catalyzed C(sp2)−H/C(sp3)−H coupling of (vinyl)arenes with α-diazoketones is reported, wherein the diazo group was exploited as a traceless auxiliary for control of regioselectivity. Challenging β-(hetero)aryl/alkenyl ketones were obtained through this operationally simple approach. The cascade process merges denitrogenation, carbene rearrangement, C−H activation, and hydroarylation/hydroalkenylation. The robustness of this method was demonstrated at preparative scale and applied to late-stage diversification of natural products.
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Q. Lu, S. Cembellín, S. Greßies, S. Singha, F. Glorius.* ___________________________________________________________________________
An unprecedented MnI-catalyzed (2-indolyl)methylation of heteroarenes is reported. This method makes use of an aromatizing cascade strategy to install a (2-indolyl)methyl group into target molecules, thereby leading to the expedient synthesis of previously challenging and important unsymmetrical diheteroarylmethanes, in particular bis(2-indolyl)methanes. The proposed cascade process comprises the reorganization of multiple bonds with controlled regioselectivity and high atom economy and can be performed on a gram-scale. Furthermore, a metal-free C−H propargylation is observed. The diverse application of this method is also demonstrated.
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Q. Lu#, S. Greßies#,
S. Cembellín, F. J. R. Klauck, C. G. Daniliuc, F. Glorius.* ___________________________________________________________________________
A strategy is reported in which traceless directing groups (TDGs) are used to promote the redox-neutral MnI-catalyzed regioselective synthesis of N-heterocycles. Alkyne coupling partners bearing a traceless directing group, which serves as both the chelator and internal oxidant, were used to control the regioselectivity of the annulation reactions. This operationally simple approach is highly effective with previously challenging unsymmetrical alkyne systems, including unbiased dialkyl alkynes, with perfect regioselectivity. The simple conditions and the ability to carry out synthesis on a gram scale underscore the usefulness of this method. The application of this strategy in the concise synthesis of the bioactive compound PK11209 and the pharmaceutical moxaverine is also described.
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Q. Lu, S. Greßies,
F. J. R. Klauck, F. Glorius.* ___________________________________________________________________________
A MnI-catalyzed regioselective C−H allenylation is reported that allows a broad range of 2-allenylindoles to be synthesized regioselectively on a gram scale under simple conditions. Notably, a highly efficient chirality transfer was observed (up to 93 % ee) in this transformation. This procedure was further found to allow, for the first time, the direct preparation of ketones by MnI-catalyzed C−H activation. Mechanistic investigations revealed that the precoordination of the oxygen atom to the manganese center as well as the congested tertiary carbon atom in the propargylic carbonates play a crucial role.
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Q. Lu, F. J. R. Klauck, F. Glorius.* _________ __________________________________________________________________
Manganese-catalyzed sequential C–H and C–C/C–Het bond activation to synthesize allylic alcohols, allylated arenes, functionalized cyclopentenes and skipped dienes is reported. This protocol can be readily scaled up and various coupling partners are applied in manganese catalysis for the first time. Moreover, manganese-catalyzed alkenyl C(sp2)–H activation is also shown. Complimentary to the standard solution-based protocols, these reactions also proceed efficiently under neat conditions, which is unprecedented for abundant metal catalyzed C–H activation reactions.
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H. Wang, Q. Lu, C. Chiang, Y. Luo, J. F. Zhou, G. Y. Wang, A. Lei,* ___________________________________________________________________________
Direct radical additions to terminal alkynes have been widely employed in organic synthesis, providing credible access to the anti-Markovnikov products. Because of the Kharasch effect, regioselective control for the formation of Markovnikov products still remains a great challenge. Herein, we develop a transition-metal-free, visible light-mediated radical addition of S-nucleophiles to terminal alkynes, furnishing a wide array of α-substituted vinyl sulfones with exclusive Markovnikov regioselectivity. Mechanistic investigations demonstrated that radical/radical cross-coupling might be the key step in this transformation. This radical Markovnikov addition protocol also provides an opportunity to facilitate the synthesis of other valuable α-substituted vinyl compounds.
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Q. Lu, F. Glorius.* _________ __________________________________________________________________
Radical style: Several breakthroughs have recently been achieved in enantioselective C(sp3)−H functionalization through radical activation. These new strategies show how radical chemistry can be used to convert alkanes into functionalized chiral molecules (see scheme; HAT=hydrogen atom transfer).
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P. Peng, Q. Lu, L. Peng, C. Liu, G. Wang, A. Lei.* _________ __________________________________________________________________
The dioxygen-induced radical oxyphosphorylation of alkenes and alkynes is presented, wherein a P–H bond was activated by molecular oxygen. Various β-oxy phosphonates could be facilely synthesized without the assistance of any transition metals or extra organic initiators. Mechanistic studies showed that HP(O)Ph2 acts as a reductant to accelerate oxyphosphorylation.
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H. Wang, G. Wang, Q. Lu, C. Chiang, P. Peng, J. F. Zhou, A. Lei.* _________ __________________________________________________________________
Difunctionalization of activated alkenes, a powerful strategy in chemical synthesis, has been accomplished for direct synthesis of a series of β-keto sulfides and β-keto sulfones. The transformation, mediated by O2, proceeds smoothly in water and without any catalyst. Prominent advantages of this method include mild reaction conditions, purification simplicity, and gram-scale synthesis, underlining the practical utility of this methodology.
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Z. Huang#, Q. Lu#, Y. Liu, D. Liu, J. Zhang, A. Lei.* _________ __________________________________________________________________
A highly regio- and stereoselective oxysulfonylation of allenes was developed that provided direct access to 2-sulfonyl allylic alcohols in good yields. By means of dioxygen activation, selective difunctionlization of allenes could be successfully achieved under mild metal-free conditions. Preliminary mechanistic investigation disclosed that this transformation probably goes through a radical process.
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Q. Lu, S. Vásquez-Céspedes, T. Gensch, F. Glorius.* _________ __________________________________________________________________
Achieving controllable C–H functionalization to elaborate valuable compounds from simple chemicals is attractive and highly desirable, especially if nonprecious transition metal catalysts can be used. However, controlling selectivity in these transformations remains a continuous challenge to synthetic chemists. Herein, we show for the first time that control over the reactive organometallic intermediate enables the switchable synthesis of quinoline and indole from amides and alkynes through C–H activation using Cp*Co(III). The keys to this strategy are (1) introducing a Lewis acid to greatly accelerate the dehydrative cyclization, which can outcompete dehydrogenative cyclization, and (2) tuning the directing group to facilitate the dehydrogenative cyclization and inhibit dehydrative cyclization.
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C. Liu, Q. Lu, Z. Y. Huang, J. Zhang, F. Liao, P. Peng, A. Lei.* ___________________________________________________________________________
A novel strategy was developed to trigger ·CF3 by using in situ generated peroxide in NMP under O2 or air as the radical initiator. Radical trifluoromethylation of alkenes was achieved toward tertiary β-trifluoromethyl alcohols. Various tertiary β-trifluoromethyl alcohols can be synthesized in good yields without extra oxidants or transition metal catalysts. Preliminary mechanistic investigation revealed that O2 diffusion can influence the reaction rate.
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H. Wang#, Q. Lu#, C. Qian, C. Liu, W. Liu, K. Chen, A. Lei.* ___________________________________________________________________________
Controlling selectivity is of central importance to radical chemistry. However, the highly reactive and unstable radical intermediates make this task especially challenging. Herein, a strategy for taming radical redox reactions has been developed, in which solvent-bonding can alter the reactivity of the generated radical intermediates and thereby drastically alter the reaction selectivity at room temperature. Various β-oxy sulfoxides and β-hydroxy sulfides can be facilely obtained, some of which are difficult to synthesize by existing methods. Notably, neither a metal catalyst nor any further additives are necessary in these processes.
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Q. Lu#, P. Peng#, Y. Luo, Z. Zhao, M. Zhou, A. Lei.* _________ __________________________________________________________________
A copper-catalyzed ketooxygenation of electron-deficient alkenes was developed. This approach combines O-H alkylation, aerobic decarboxylation, and oxygenation in one transformation. Mechanistic investigation of this reaction showed that the copper salt is responsible for both generating the amidoxyl radical and promoting aerobic decarboxylation.
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Q. Lu, Z. Liu, Y. Luo, G. Zhang, Z. Huang, H. Wang, C. Liu, J. T. Miller, A. Lei.* _________ __________________________________________________________________
A highly selective radical dioxygenation of alkenes using hydroxamic acid and O2 was developed, and copper/cobalt was used as the catalyst without assistance of any additional ligands or bases. Mechanistic investigation disclosed that copper salt and O2 work in concert to activate hydroxamic acid, with Cu(I) and Cu(II) concurrently existing in this reaction.
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Q. Lu#, J. Zhang#, P. Peng, G. Zhang, H. Zhang, H. Yi, J. T. Miller, A. Lei.* _________ __________________________________________________________________
An unprecedented single electron redox process in copper catalysis is confirmed using operando X-ray absorption and EPR spectroscopies. The oxidation state of the copper species in the interaction between Cu(II) and a sulfinic acid at room temperature, and the accurate characterization of the formed Cu(I) are clearly shown using operando X-ray absorption and EPR evidence. Further investigation of anion effects on Cu(II) discloses that bromine ions can dramatically increase the rate of the redox process. Moreover, it is proven that the sulfinic acids are converted into sulfonyl radicals, which can be trapped by 2-arylacrylic acids and various valuable β-keto sulfones are synthesized with good to excellent yields under mild conditions.
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Q. Lu, H. Wang, P. Peng, C. Liu, Z. Huang, Y. Luo, A. Lei.* _________ __________________________________________________________________
A new reaction for O2 fixation is rationally demonstrated, in which radical thiolation/oxygenation of activated alkenes proceeds spontaneously at room temperature with no need for any additives. EPR and operando IR experiments revealed that O2 serves as the initiator to trigger this radical reaction and that autoinductive processes are the critical factor to drive the reaction.
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Q. Lu, J. Chen, C. Liu, Z. Huang, P. Peng, H. Wang, A. Lei.* _________ __________________________________________________________________
Autoxidative carbon–heteroatom bond formation using activated alkenes as a linkage is described. Heteroatom (O, S) nucleophiles could be transformed into different kinds of valuable β-keto compounds via an O2-mediated C(sp2)–X bond oxygenation process, without using any external organic oxidants or metal catalysts.
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Q. Lu, H. Yi, A. Lei.* _________ __________________________________________________________________
As a highly economic and efficient strategy for chemical bond formation, oxidative coupling has been widely applied in synthetic organic chemistry. Recently, Klussmann, Jiao and Huo et al. groups have successfully achieved the direct functionalization of Csp3-H and Csp2-H bonds through developing simple, elegant autoxidative coupling reactions, which makes a significant progress in this filed. This highlight mainly reviews the recent advances in autoxidative coupling.
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J. Zhang, Q. Lu, C. Liu, A. Lei.* ___________________________________________________________________________
Recently, transition-metal-catalyzed oxidative coupling reactions have emerged as the most important and attractive methods to construct carbon-carbon bond and carbon-heteroatom bonds in organic synthetic chemistry. This review mainly summarizes our research progress in the area of oxidative coupling and their versatility, application in organic synthesis, especially for the third generation of oxidative coupling.
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Q. Lu#, C. Liu#, Z. Huang, Y. Ma, J. Zhang, A. Lei.* ___________________________________________________________________________
A new radical oxytrifluoromethylation of alkenes via an aerobic Cvinyl–heteroatom bond oxygenation process is reported, in which O2 and a catalytic amount of K2S2O8 work in concert to activate CF3SO2Na. Mechanistic investigation disclosed that CF3SO2˙ could react with O2 to reinitiate radical chain process.
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Q. Lu, C. Liu, P. Peng, Z. Liu, L. Fu, J. Huang, A. Lei.* _________ __________________________________________________________________
A copper-catalyzed radical oxidative trifluoromethylation and arylation of electron-withdrawing alkenes has been developed, in which economical sodium trifluoromethanesulfinate is used as the CF3 source. A preliminary kinetic investigation indicated that the generation of CF3 radicals under the mild conditions might be a facile process.
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Q. Lu, J. Zhang, G. Zhao, Y. Qi, H. Wang, A. Lei.* ___________________________________________________________________________
An unprecedented dioxygen-triggered oxidative radical process was explored using dioxygen as the solely terminal oxidant, realizing aerobic oxidaitve difunctionalization of terminal alkynes toward β-keto sulfones with high selectivity. Operando IR experiments revealed that pyridine not only acts as a base to successfully surpress ATRA (atom transfer radical addition) process, but also plays a vital role in reducing the activity of sulfinic acids.
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Q. Lu, J. Zhang, F. Wei, Y. Qi, H. Wang, Z. Liu, A. Lei.* ___________________________________________________________________________
New channel! A novel and attractive dioxygen activation by sulfinic acids was explored that is capable of performing efficiently without the assistance of transition metals or radical initiators. This reaction furnishes secondary and tertiary β-hydroxysulfones under mild conditions; β-hydroperoxysulfone was isolated as an important intermediate.
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G.-W. Wang,* Q.-Q. Lu, and J.-J. Xia,** ___________________________________________________________________________
The apparently simple Michael reaction of dimedone with chalcones afforded three types of products. The reaction of dimedone with chalcones in water at reflux, mediated by cetyltrimethylammonium bromide, gave the expected Michael adducts in good to excellent yields after purification by column chromatography. The attempted purification of the Michael adducts by recrystallization from ethanol in air unexpectedly generated hydroperoxidated and hydroxylated Michael adducts in low yields. The efficient synthesis of hydroperoxidated Michael adducts could be achieved in nearly quantitative yields by autoxidation of the Michael adducts in 1,2-dichloroethane at room temperature under aerobic conditions. The conversion of the hydroperoxidated Michael adducts to hydroxylated Michael adducts was facilitated by reduction with triphenylphosphane in excellent yields. Some of the reported Michael adducts in the literature need to be reassigned. Particular caution must be exerted to assign the Michael products generated from cyclic β-diketones, such as dimedone, because different purification processes may give different types of products.
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